The present invention relates to optical measuring apparatus which measures the thickness, diameter, circumference or other dimensional parameters of articles by scanning the surface of the article; and is particularly directed to a device for use with optical measuring apparatus to enable such optical measuring apparatus to make more accurate measurements of transparent articles or of the main profile of an article having surface anomalies projecting from the main profile.
In the prior art there are optical measuring apparatus which measure a dimension of an object or article without making physical contact with the article because they may use a scanning light beam from a light source, such as a laser, to make the desired measurement or measurements.
For instance, in U.S. Pat. No. 3,765,774, the disclosed optical measuring apparatus, with which the device of the present invention may be particularly suited to be used, has a laser light source which produces a narrow light beam having a diameter of approximately 1 millimeter. According to the description in the patent, the light beam is converted into a rotary scanning light beam by a mirror positioned within the path of the light beam at an angle of 45.degree. and mounted on a flywheel driven by a synchronous motor. The motor receives a power supply from a high frequency pulse generator or clock through an adjustable frequency divider. The rotary scanning light beam is directed through a scanner lens which converts the rotary scanning light beam to a parallel scanning light beam, and the article to be measured is located at approximately the focal point of the lens where the diameter of the light beam is minimized. On the other side of the article to be measured, and also within the path of the light beam, is another lens, the receiver lens, which converges or focuses the parallel scanning light beam onto a photodetector, which produces pulses or signals when the light reaching the photodetector changes in intensity. The signals are amplified and transmitted to a decoder which incorporates means for selecting different combinations of the signals according to the dimension to be measured. The decoder transmits the selected signals to a gate which also receives the high frequency pulses from the clock, and the output of the gate is transmitted to a pulse counter calibrated so that each counted pulse represents a finite unit of length such as 0.0005 inch. The output of the pulse counter controls a digital read-out display.
In brief, the parallel scanning light beam scans between two known reference points, and when an article is placed within the path of the parallel scanning light beam, the blackout time of the light beam, as sensed by the photodetector, corresponds precisely to the dimension of the article represented by the interruption of the parallel scanning light beam. In other words, the interrupted beam is collected by the receiver lens and focused onto a photodetector which converts the light signal to a time dependent signal. The time dependent signal is processed by appropriate electronic circuitry as shown in the patent, to give the desired read-out. The object or article to be measured may be stationary or moving as, for example, for monitoring the diameter of a wire being drawn or extruded.
Other measurements or monitorings may include those involving the diameters of rods and cylinders; the thickness of a paper web; the thickness of a plastic insulation coating on a wire; monitoring inside and outside diameters of annular articles, glass tubing and rubber hose; measuring or monitoring steel, copper, brass tubing and profiles; making hole measurements; making length measurements; and measuring or monitoring belt or sheet thickness for rolling mills.
Any physical measurement including length, width, diameter, thickness may be made where a beam of light can be used to differentiate the beginning and end of the dimension.
Other embodiments of the optical measuring apparatus of the above-mentioned patent are disclosed in U.S. Pat. Nos. 3,905,705 and 4,007,992.
In the manufacture of tobacco smoke cigarette filter rods, one of the primary physical properties which must be controlled is the circumference. Traditionally, filter rod circumference has been measured by means of pneumatic gauges, which are well known in the industry. With the growth of low tar cigarettes, filter rods wrapped with porous plugwrap have become increasingly popular. The circumference of porous wrapped filter rods, however, cannot be measured accurately with the traditional pneumatic gauges because of air penetration into the porous wrap. When the sense air penetrates the porous wrap, there is a loss of back pressure, the air being lost outside the plenum chamber. For instance the sense air upon entering the porous rod will progress along the rod axis and exit either through the porous wrap outside the circumference head of the instrument involved, or exit the tip of the filter rod itself.
In attempting to use laser scanning apparatus such as disclosed in the three patents above, there is a loss of accuracy when measuring the circumference of porous wrapped tobacco smoke filter rods or when measuring diameters, circumferences, etc., of non-wrapped filter-type rods, for whatever their uses may be. The errors are caused by two factors. The first factor is the surface fuzz associated with porous wraps and the non-wrapped cellulose acetate rods. A fuzzy surface on a filter rod tends to cause the optical measuring apparatus to overstate the circumference because the surface fibrils of the porous wrap project above the main profile of the filter rod. This situation was proved when a metal standard for the filter rod was used and then lint was applied to the surface of the metal standard. The optical measuring apparatus read higher than the actual metal standard value. The second factor which has contributed to error has been that of the seam lip on the porous wrapped filter rod, the distance from the edge of the seam to the surface of the rod.
Other instances where fuzzy surfaces or other types of surface irregularities projecting from the main profile of the object or article to be measured or monitored may cause error include the fuzzy surface of paper webs, and water droplets or other liquid droplets on the surfaces of wire or plastic or otherwise insulated wire as it is being drawn or extruded.
In the instance of using optical instruments for measuring or monitoring glass rods which are transparent, certain errors may be induced because part of the scanning light beam may pass through the transparent glass surface or through a portion thereof rather than be totally interrupted by the presence of the transparent article within the light beam scanning path.
The above-mentioned U.S. Pat. No. 4,007,992 represents one solution for trying to avoid surface irregularities. This patent is directed to an optical measuring apparatus where the scanning light beam is shaped to be wider than the dimension of any single particle of foreign material so as to be greater than the dimension or irregularities on the surface of an article. When the light beam is scanned across the article an average contour is sensed, i.e., the light beam is greater in magnitude than any individual particle of foreign material upon the surface of the object. In this manner only a portion of the light beam is interrupted by the surface particle and a major portion of the beam passes the particle on the article. This solution may be effective for isolated foreign particles on the surface of an article, but is not effective when there are so many surface fibrils projecting from the main profile of the article that the apparatus determines that the main profile is greater in dimension that it actually is.
An object, therefore, of the invention is to provide a device that may be readily attached to an apparatus such as the optical measuring apparatus disclosed in the aforementioned U.S. Pat. Nos. 3,765,774 and 3,905,705, or to other types of optical measuring or monitoring apparatus so as to enable such apparatus to be more accurate and effective in measuring transparent articles or in measuring the main profile of an article having surface irregularities or anomalies projecting from such main profile.
Other objects of the invention will become apparent to those skilled in the art to which this invention pertains from the description which follows.